Probe station thermal chuck with shielding for capacitive current

ABSTRACT

To reduce noise in measurements obtained by probing a device supported on surface of a thermal chuck in a probe station, a conductive member is arranged to intercept current coupling the thermal unit of the chuck to the surface supporting the device. The conductive member is capacitively coupled to the thermal unit but free of direct electrical connection thereto.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 09/345,571, filed Jun.30, 1999, now U.S. Pat. No. 6,445,202 B1.

BACKGROUND OF THE INVENTION

The present invention is directed to probe stations suitable for makinglow current and low voltage measurements and, more particularly, to asystem for reducing noise due to capacitive currents resulting from theoperation of a thermal chuck for a probe station.

Integrated circuit devices are typically manufactured in and on a waferof semiconductor material using well-known techniques. Prior to cuttingthe individual integrated circuit devices from a wafer, tests are run onindividual devices to determine if the devices operate properly. Thewafer is supported on a chuck inside an environmental enclosure in aprobe station. Probes are brought into contact with test points or padson the integrated circuit devices and a series of measurements arepreformed. Schwindt et al., U.S. Pat. No. 5,663,653, disclose an exampleof a probe station in which the present invention might be used and thepatent is incorporated herein by reference.

Many integrated circuit devices are designed to operate at temperaturesother than room temperature. To accommodate device testing attemperatures other than the ambient temperature, a thermal chuck may beemployed. One design of a thermal chuck comprises a multilayered chuckfor securing a wafer having a thermal driver to modify the temperatureof the chuck. A thermal chuck of this design is disclosed by Schwindt inU.S. Pat. No. 5,610,529 which is incorporated herein by reference.

The thermal driver may provide for either heating, cooling, or heatingand cooling of the chuck. To modify the temperature of the chuck, thethermal driver may comprise one or more thermal units including athermal device and a plurality of power conductors connecting thethermal device to a power source. Thermal devices, typically electricresistance heaters or thermoelectric heat pumps, are provided to heatthe chuck to temperatures above the ambient temperature. Thethermoelectric heat pump, also known as a Peltier device, is reversibleand can be used for cooling as well as heating the chuck. Thethermoelectric heat pump comprises a number of thermocouples sandwichedbetween two electrically insulating, thermally conductive plates. WhenDC power is supplied to the thermocouples, the Peltier effect causesheat to be transferred from one plate to the other. The direction ofheat flow is reversible by reversing the direction of current flow inthe thermocouples. Exposing the chuck to the warmer plate or the coolerplate of the thermoelectric heat pump will, respectively, either heat orcool the chuck. For testing at temperatures below ambient, the thermalchuck may also include passages for circulating coolant to cool thechuck directly or remove excess heat from the thermoelectric heat pump.

When making the low voltage and low current measurements common totesting integrated circuit devices, even very low levels of electricalnoise are unsatisfactory. Thermal chucks include several sources ofnoise and unacceptably high levels of noise are a common problem whenusing a thermal chuck. One known source of noise is the result ofexpansion or contraction of the components of the thermal chuck due tochanging temperature. Expansion or contraction changes the spacingbetween conductive components resulting in the generation of capacitivecurrents which can reach the conductive surface of the chuck. Expansionor contraction due to temperature change can also cause relativetransverse movement between the multiple material layers of the chuck.Relative movement between contacting layers of insulating and conductivematerials can generate triboelectric current. In a probe station chuck,the triboelectric current can appear as noise in the test measurements.Triboelectric currents can be reduced by a chuck design which preventsmovement between contacting layers of insulating and conductingmaterials.

The operation of the thermal units by the thermal driver controller isanother potential source of noise when using a thermal chuck. To changeor maintain the temperature of the thermal chuck, the thermal drivercontroller fluctuates the electrical power to the thermal units inresponse to a temperature control system. As a result of the voltagedrop within the conductors of the thermal units, physically adjacentportions of the electrical conductors leading to and from, and internalto the thermal devices, will be at different potentials. As the powerfluctuates, the difference in voltage between the power conductorschanges with time. This results in a displacement of charges in thedielectric material surrounding the conductors which manifests itself asa displacement or capacitive current coupled to the conductive topsurface of the chuck. This capacitive current appears as noise in thetest measurements.

The currently accepted technique to reduce the effects of capacitivecurrents involves shielding the chuck from external electromagneticsources. However, the shielding layers of conductive material in thechuck have proven unsuccessful in eliminating the noise from the thermaldriver. To reduce noise due to capacitive currents originating in thethermal chuck, users of probe stations often shut off the thermal unitsand wait for the current to dissipate. However, the RC time constantinvolved can be greater than five seconds. Waiting a period of five timeconstants (e.g. 25 seconds) for the observed noise to dissipate to anacceptable level before making a measurement substantially effects theproductivity of the probe station.

What is desired, therefore, is a system for reducing the electricalnoise generated by the operation of the thermal unit of a probestation's thermal chuck. Reducing noise generated by the thermal chuckreduces the time for the noise to dissipate to acceptable levelsimproving the productivity of the probe station.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of a probe station incorporating a thermalchuck.

FIG. 2 is a cross section of an exemplary thermal chuck constructed inaccordance with the present invention.

FIG. 3 is an exemplary schematic diagram of a thermal unit and shieldingin accordance with a first aspect of a preferred embodiment of thepresent invention.

FIG. 4 is an exemplary schematic diagram of a thermal unit and shieldingin accordance with a second aspect of a preferred embodiment of thepresent invention.

FIG. 5 is an exemplary schematic diagram of a thermal unit and shieldingin accordance with a third aspect of a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As illustrated in FIG. 1, a probe station generally includes anenvironmental enclosure 2 in which is located a chuck 4 and one or moreprobes 6. The environmental enclosure 2 is typically constructed of aconductive material and grounded 7 so that the chamber, interior to theenclosure 2, is shielded from electromagnetic fields emanating fromoutside of the enclosure 2. The chuck 4 typically comprises multiplelayers of conductive and dielectric materials that are connected to thevarious conductors of a coaxial or triaxial cable 8. The chuck 4includes a securement technique for securing a device under test 10,generally a wafer of semiconductor material, to the upper surface 12 ofthe chuck 4. The upper surface 12 of the chuck 4 is typicallyconductive. One technique for securing a device under test 10 relies ona vacuum source (not shown) located outside of the environmentalenclosure. The vacuum source communicates through appropriate controlvalves and piping with apertures (not shown) in the upper surface 12 ofthe chuck 4. When the device under test 10 is placed on the chuck 4 thedevice blocks apertures leading to the vacuum source. Air pressure holdsthe device under test 10 against the chuck's upper surface 12. One ormore probes 6 can be positioned over the device under test 10 andbrought into contact with test pads on the circuit to be tested.Instrumentation connected to the probes 6 measures selected operatingparameters of the circuit at the test pads.

A thermal chuck 14, bracketed, may be used to test the operation ofdevices at temperatures other than the ambient temperature of theenvironmental enclosure 2. Referring to FIG. 2, the thermal chuck 14,indicated with a bracket, may include a thermal driver 16 havingfacilities for modifying the temperature of a chuck 4, indicated with abracket, supported on the top of the thermal driver 16. The thermaldriver 16 may be arranged to provide for either heating, cooling, orheating and cooling of the chuck 4. The thermal driver 16 comprises oneor more electrically powered thermal units 20 each of which includes oneor more thermal devices 22 and a plurality of insulated power conductors24 connecting the thermal devices 22 to a thermal driver controller 18.Typically, the thermal devices 22 are resistance heaters orthermoelectric heat pumps. Resistance heaters and thermoelectric heatpumps can increase the temperature of the chuck 4. The thermoelectricheat pump can also be used to cool the chuck 4. The thermoelectric heatpump, also known as a Peltier device, comprises a plurality ofelectrically connected thermocouples of p-type and n-type semiconductormaterials sandwiched between two plates of an electrically insulating,thermally conducting material. When DC power is supplied to thethermocouples, heat is transferred from one plate to the other as aresult of the Peltier effect. The direction of heat flow is reversibleby reversing the direction of current flow in the semiconductors.Exposing the chuck 4 to the warmer plate or the cooler plate of thethermoelectric heat pump will, respectively, heat or cool the chuck 4.

The thermal driver 16 may also include passages 26 for circulatingcoolant supplied by a coolant source (not shown) typically locatedoutside of the environmental enclosure 2. For testing at temperaturesbelow the ambient temperature, the chuck 4 may be cooled directly by thecoolant. If a thermoelectric heat pump is used to cool the chuck,circulating coolant may be necessary to remove heat transferred to thethermal driver 16 by the heat pump.

Electric power for the thermal units 20 is supplied by the thermaldriver controller 18 located outside of the environmental enclosure 2.Insulated power conductors 24 transfer the electrical power to thethermal devices 22 in the thermal chuck 14. In response to a temperaturesensing system, the thermal driver controller 18 fluctuates theelectrical power to the thermal unit 20 to vary its thermal output toeither reduce or increase the rate of addition or removal of thermalenergy to or from the chuck 4. As a result of the voltage drop in thethermal unit 20, adjacent portions of the insulated power conductors 24and the conductors inside the thermal devices 22 are at differingpotentials. This causes a displacement of charge in the dielectricmaterial surrounding the conductors. As the thermal driver controller 18fluctuates the power to the thermal unit 20 the difference in voltagebetween adjacent conductors also varies with time. The present inventorscame to the realization that this displacement of charge varying withtime causes a displacement or capacitive current which is coupled to theconductive upper surface 12 of the chuck 4. The present inventorsfurther realized that this capacitive current manifests itself as noisein the test measurements.

The present inventors came to the realization that the aforementionedcapacitive currents are a significant source of noise when makingmeasurements in the femtoamp range with state of the art probe stations.The present inventors further realized that conductive shielding of thethermal unit 20 that is capacitively coupled to the conductors of thethermal unit 20 can intercept a substantial amount, and preferablysubstantially all, of the capacitive currents resulting from theoperation of the thermal unit 20 and provide a conductive path to returnany current induced in the conductive shielding to the thermal drivercontroller 18 and to ground. This is in contrast to the presentlyaccepted techniques of adding more shielding to the chuck itself.Referring also to FIG. 3, a conductive thermal device shell 28substantially encloses the thermal devices 22 and the power conductors24 at their connection to the thermal devices 22. Variation in chargedisplacement resulting from the operation of the electric circuit of thethermal device 22 results in a displacement current in the conductivethermal device shell 28. In other words, the thermal device shell 28 iscapacitively coupled through “virtual” coupling capacitors 30 to theelectric circuit of the thermal device 22 and intercepts capacitivecurrents that would otherwise find their way to the upper surface 12 ofthe chuck 4. Although apertures may be required in the thermal deviceshell 28 they should be minimized in relation to the total surface areaof the thermal device shell 28. The more completely the thermal deviceshell 28 spatially encloses the thermal device 22 the more completely itwill intercept capacitive currents emanating from the thermal device 22.The thermal device shell 28 is conductively connected to the thermaldriver controller 18 through the conductive shield of the cable 32. Theconductive connection of the thermal device shell 28 to the thermaldriver controller 18 provides a path for any current in the thermaldevice shell 28 to exit the environmental enclosure 2 to the thermaldriver controller 18. The driver controller 18 is connected to ground 7extending the conductive return path for capacitive currents to ground7.

The present inventors also came to the stark realization that byenclosing the thermal devices 22 with a conductive shell 28 the RC timeconstant of the thermal chuck is dramatically reduced. The thermaldevices 22 do not need to be turned off in order for the noise to besufficiently reduced. The present inventors determined that thisreduction in RC time constant is due to a reduction in the storedcapacitive charge in the dielectric material within the chuck, referredto as absorption capacitance. The absorption capacitance of a materialincludes a series resistance so, in effect, it has a memory of previouscharges and is slow to dissipate. This absorption capacitance was notpreviously considered in the design of thermal chucks. There was little,if any, motivation to enclose the thermal devices 22 in a conductiveenclosure, as it was believed that noise from the thermal devices 22could be removed by layers of shielding in the chuck 4. The layers ofthe chuck 4 include, however, dielectric material which the inventorrealized is, in fact, a source of the long RC time constant.

The cable 32 includes the power conductors 24 connecting the thermaldriver controller 18 to the thermal devices 22. The shield of the cable32 ideally extends through the wall of the environmental enclosure 2 andencompasses the power conductors 24 at their entrance into the thermaldevice shell 28. The shield of the cable 32 is capacitively coupled tothe power conductors 24 and will intercept and return to the thermaldriver controller 18 currents emanating from the capacitive effects ofpower fluctuation in the power conductors 24. The thermal drivercontroller 18 is grounded at ground connection 21. The more complete theenclosure of all conductors in the thermal unit 20 by the conductiveshielding, the more complete will be the protection of the testmeasurement from noise generated by the operation of the thermal unit20.

The walls of the environmental enclosure 2 are typically conductivematerial. The conductive material shields the chamber inside theenvironmental enclosure 2 from electromagnetic (EM) fields originatingoutside of the enclosure 2 which would otherwise result in noise withinthe probe 6. The environmental enclosure 2 is grounded to return toground the currents generated in the conductive wall by the EM fields.In a preferred embodiment of the present invention, the conductive wallof the environmental enclosure is extended to substantially surroundparts of the thermal units. The extension of the wall of the enclosureprovides a conductive shield capacitively coupled to the thermal unitswhich can return capacitive currents to the enclosure ground.

Referring to FIG. 3, in a first aspect of this preferred embodiment thewall of the environmental enclosure 2 is extended coaxially with yetanother shield layer 34 of the cable 32 to a point of close physicalproximity to the thermal device shell 28 yet being free from directelectrical connection to the shield of the cable 32, the thermal drivercontroller 18, and the thermal device shell 28. The wall of theenvironmental enclosure 2 is extended proximate to the thermal deviceshell 28 by connecting the outer shield layer 34 of the cable 32 to thewall of the environmental enclosure 2. The cable 32 includes the powerconductors 24 connecting the thermal driver controller 18 to the thermaldevices 22. Capacitive currents emanating from the power conductors 24are intercepted by the shield of cable 32 and returned to the thermaldriver controller 18 and the thermal driver controller ground 21. Theextension of the wall of the environmental enclosure 2 through the outershield 34 of the power cable 32 is capacitively coupled to the shield ofthe cable 32 by a “virtual” capacitor 36 and intercepts capacitivecurrents leaking from within the cable 32 which might otherwise coupleto the chuck 4. Any current in the extension of the environmentalenclosure 2 is returned to ground 7 outside of the environmentalenclosure 2 if switch 23 is closed. If the switch 23 is open, capacitivecurrents are returned to the ground 25 of an instrument 27 which isconnected by leads 29 to probes inside the chamber.

Referring to FIG. 4, in a second aspect of this preferred embodiment thewall of the environmental enclosure 40 is extended to substantiallysurround the thermal devices 42, the thermal device shell 44 and thepower cable 46 connecting the thermal devices 42 to the thermal drivercontroller 50. Heat is transferred to and from the chuck 56 through thethermal device shell 44 and the wall of the environmental enclosure 40.The thermal devices 42 are capacitively coupled to the thermal shell 44by virtual capacitors 48. The thermal device shell 44 and the shield ofthe power cable 46 are, in turn, capacitively coupled to the wall of theenvironmental enclosure 40 by virtual coupling capacitors 52. Capacitivecurrents in the thermal device shell 44 or the shield of the cable 46are returned to the thermal driver controller 50 through the conductiveshield layer of the cable 46. The thermal driver controller 50 isconnected to the thermal devices 42 by power conductors 43 and to groundat ground 51. Capacitive currents leaking from the thermal device shell44 or the power cable 46 will be intercepted by the wall of theenclosure 40 and returned to the enclosure ground 54 when the switch 53is closed. When the switch 53 is open, capacitive currents in the wallof the environmental enclosure 40 are returned to the ground 55 ofinstrument 57. The instrument 57 is connected to the probes 6 inside theenvironmental enclosure by instrument leads 47.

Referring to FIG. 5, in a third aspect of this preferred embodiment thewall of the environmental enclosure 60 is extended to surround thethermal devices 64 and the power conductors 62 connecting the thermaldevices 64 to the thermal driver controller 63. The thermal drivercontroller is grounded at ground 74. In this aspect of the invention,the thermal devices 64 and the power conductors 62 are capacitivelycoupled to the wall of the environmental enclosure 60 through thevirtual coupling capacitors 66. Capacitive currents generated in thethermal devices 64 or power cables 62 are intercepted by the shieldformed by the conductive wall of the enclosure 60 and returned to theenclosure ground 68 when the switch 69 is closed. If the switch 69 isopen the walls of the enclosure 60 are grounded through the instrument73 to the instrument ground 71. Heat is transferred to and from thechuck 70 through the wall of the environmental enclosure 60.

The terms and expressions that have been employed in the foregoingspecification are used as terms of description and not of limitation,and there is no intention, in the use of such terms and expressions, ofexcluding equivalents of the features shown and described or portionsthereof, it being recognized that the scope of the invention is definedand limited only by the claims that follow.

What is claimed is:
 1. A method of reducing measurement noise duringprobe testing of a device under test supported on a surface of a chuck,said chuck including a thermal unit for modifying a temperature of saiddevice under test, said method comprising the steps of: (a) arranging aconductive member to intercept a current coupling said thermal unit tosaid surface of said chuck, said conductive member being capacitivelycoupled to said thermal unit while free of direct electrical connectionthereto; and (b) conductively connecting said conductive member to acontroller supplying power to said thermal unit.
 2. The method of claim1 wherein the step of arranging a conductive member to intercept acurrent coupling said thermal unit to said surface of said chuckcomprises the step of substantially enclosing said thermal unit in aconductor, said conductor including a surface interposed between saidthermal unit and said surface of said chuck.
 3. The method of claim 1further comprising the step of grounding said conductive memberproximate to said controller.
 4. A method of reducing measurement noiseduring probe testing of a device under test supported on a surface of achuck, said chuck including a thermal unit for modifying a temperatureof said device under test, said method comprising the steps of: (a)arranging a first conductive member to intercept a current coupling saidthermal unit to said surface of said chuck, said first conductive memberbeing capacitively coupled to said thermal unit while free of directelectrical connection thereto; (b) arranging a second conductive membersubstantially encircling a major length of a power conductor connectingsaid thermal unit and a controller supplying power to said thermal unit;and (c) conductively connecting said first and said second conductivemembers to said controller.
 5. The method of claim 4 wherein the step ofarranging a first conductive member to intercept a current coupling saidthermal unit to said surface of said chuck comprises the step ofsubstantially enclosing said thermal unit in a conductor, said conductorincluding a surface interposed between said thermal unit and saidsurface of said chuck.
 6. The method of claim 4 further comprising thestep of grounding said first and said second conductive membersproximate to said controller.
 7. A method of reducing measurement noisewhen probe testing a device under test supported on a surface of achuck, said chuck including a thermal unit for modifying a temperatureof said device under test, said method comprising the steps of: (a)substantially enclosing said thermal unit and a conductor of power tosaid thermal unit in a conductive enclosure, said conductive enclosureincluding a surface interposed between said surface of said chuck andsaid thermal unit and interposed between said surface of said chuck anda substantial length of said conductor; and (b) conductively connectingsaid conductive member to a controller supplying power to said thermalunit.
 8. The method of claim 7 further comprising the step of groundingsaid conductive member proximate to said controller.